Any system transmitting data in a mobile or portable radio environment is subject to both random and burst errors, requiring the use of an error detection and correction technique which provides protection from both.
Many coding schemes exist wherein data which is to be transmitted over a wireline or over a radio link is encoded in such a fashion as to reduce the number of errors received at a receiving user's terminal. Data to be transmitted is usually conveniently arranged into groups of bits representing a character. Error codes include parity bits sent with each group of bits, which in their most elementary form indicate whether one bit of the group of bits is in error, without determining which one, to much more powerful coding techniques which not only detect errors, but which also allow correction of multiple bit errors.
Since the strength of the signal transmitted (and received) affects the number of errors in the received signal, the degree of protection provided by the error detection and correction technique determines the required transmit power levels used, and in turn has a significant impact on portable terminal battery requirements and portable terminal size and weight. The more robust the error detection and correction algorithm employed, the smaller and lighter the portable terminal can be.
Error detection and correction codes typically comprise the use of a rearrangement in the bit sequences of the message in order to limit the damage to the received message due to a "burst error" or an error which destroys several consecutive bits of data (bit interleaving, for example), or the use of additional bits transmitted with the message which may be used to determine whether errors in the received message exist, and, in some cases, to correct such errors. However, the error detection and correction algorithm must not be inefficient, or radio channel throughput will suffer. Also the algorithm should be simple enough to implement on readily available 8 bit and 16 bit microprocessors, such that the microprocessors can keep up with the data received from the channel.
Most existing portable and mobile radio data systems require a channel bit error rate of 1% or less to achieve reliable communications. To achieve such low bit error rates, data must be transmitted at relatively slow rates, which decreases the amount of information which can be transmitted on a radio channel (or on a single wire line in the case of a landline data transmission network), or the power of the transmitter must be increased to be able to overcome noise and the like which may be the cause of the errors. In the case of portable radio data systems, increased power creates significant problems with respect to power consumption, and, thereby, battery life.
If an error detection and correction scheme could be implemented which allowed reliable, accurate communications while tolerating a bit error rate of 10%, such a system would allow reduced output power by about 50% in the transmit mode, thus reducing current consumption (particularly critical for portables) by about 50% in the transmit mode. Alternatively, an increased coverage area could be obtained.
The present invention provides such an error detection and correction scheme.